Cleaning control for electrostatic precipitator

ABSTRACT

AN INDUSTRIAL ELECTROSTATIC PRECIPITATOR HAVING A RAPPER CONTROL WHICH INITIATES A RAPPER CYCLE IN RESPONSE TO DUST WEIGHT ACCUMULATIONS ON THE COLLECTOR PLATES. THE PLATES ARE DEFLECTED DOWNWARDLY BY THE WEIGHT OF THE DUST; AFTER PREDETERMINED WEIGHT INCREASE THE DEFLECTION IS SUFFICIENT TO ACTUATE A TRIGGER FOR THE RAPPER CONTROL.

I6 j 50 H 5 1 n p 21, 1.971 A. A. ARSTIKIAITIS 3,505,733

CLEANING CONTROL FOR ELECTROSTATIC PRECIPITATOR Filed July 17, 1969 22-? I8 22 laja s 1,3 Z2 [8 4;

l I l IN VENTOR.

United States Patent 3,606,733 CLEANING CONTROL FOR ELECTROSTATIC PRECIPITATOR Arunas A. Arstikaitis, Dearborn Heights, Mich., assignorto American Standard Inc., New York, NY. Filed July 17, 1969, Ser. No. 842,588 Int. Cl. B03c 3/76 U.S. Cl. 55-112 Claims ABSTRACT OF THE DISCLOSURE An industrial electrostatic precipitator having a rapper control which initiates a rapper cycle in response to dust weight accumulations on the collector plates. The plates are deflected downwardly by the weight of the dust; after predetermined weight increase the deflection is sufiicient to actuate a trigger for the rapper control.

THE DRAWINGS FIG. 1 is a schematic representation of the upper portion of an electrostatic precipitator which can be equipped with a rapper control of the present invention.

FIG. 2 is a sectional view taken through the sensortrigger portion of a rapper control of the present invention.

FIG. 3 shows another form which the sensor-trigger mechanism can take in practicing the invention.

FIG. 4 is an electric circuit diagram of a rapper control using the invention.

THE DRAWINGS IN DETAIL FIG. 1 illustrates the upper portion of an industrial electrostatic precipitator used to remove dust particles from furnaces and other equipment in such environments as power plants, kilns, steel mills, dryers, sintering and calcinating plants, etc. As shown, the precipitator comprises a housing having a sidewall 12 and a roof 14.

PARTICLE IONIZATION Disposed within the housing is a horizontal rectangular frame 16, from which are suspended suitable ionizer wires 18, each frame and its wires forming one ionizer section. In practice the precipitator may comprise many sections arranged behind and alongside one another.

The high voltage power supply for the illustrated ionizer Wires may take the form of two insulated hanger rods 20, positioned one behind the other, each rod being connected at its lower end to frame 16 to supply electric power to the frame and to the depending ionizer wires 18. In practice the wires may be arranged in rows, spaced apart about eight inches, the wires in each row being spaced from one another about seven inches; each row may contain for example twelve wires. The dusty gas is fed into the housing 10 in a direction normal to the plane of the paper so that the gas flows past the various wires 18 in each of the six illustrated rows. As is conventional, the wires may be held substantially taut by means of weights fastened to the lower ends of the wires (not shown).

COLLECTOR PLATES The rows of various taut wires are arranged between grounded collector plates 22 suitably suspended from an overhead frame 24. As seen in FIG. 1, the collector plates present their frontal edges to the entering dusty gas. In a direction normal to the paper the plates may be five feet wide and thirty feet high. Along its upper edge each plate may be reinforced with a tubular section 25; as shown in FIG. 2 pipes or the like 26 may extend into the extremities of each tubular section to suspend the individual plates from frame 24.

ice

Frame 24 may be suitably suspended within housing 10 by various structural arrangements. As shown in FIG. 1, the suspension mechanism includes a stationary channel member 28 extending across the inlet face of the housing in the space forwardly of the front edges of the collector plates 22; a similar stationary channel member (not shown) would extend across the corresponding space beyond the rear edges of the collector plates. Frame 24 may be provided with blocks or the like 30, each of said blocks being slidably arranged within a guide channel 32 secured to the rear face of channel member 28. A heavy compression spring 34 may be disposed in each channel member beneath the respective slide block, each spring thereby yieldably supporting part of the weight of frame 24 and the collector elements 22 carried thereby. In a typical arrangement the frame 24 would be equipped with four block-spring assemblies, two of them being located at the front of the frame as shown in FIG. 1, and two of them being located at the rear of the frame adjacent the non-illustrated support channel. Each spring would thus bear approximately one-fourth of the total frame-collector weight, which might be on the order of 4500 pounds.

DUST REMOVAL During operation the dust particles in the gas stream are ionized by the wires 18, and the ionized particles are attracted to the adjacent grounded collector plates 22. As the dust accumulates on the plates a resistive dust layer builds up on the plate surfaces, thus somewhat impairing the ability of the plates to attract additional dust particles. Therefore it is necessary to remove the dust accumulations from the plates. Under conventional practice such removal operations are sometimes accomplished by vibrating the plates, at frequencies of 50 cycles per second or thereabouts. More commonly the removal operations are accomplished by impulse rapping the plates at periodic intervals, as for example one or two raps every minute or thereabouts. The present invention concerns a control for either type operation, i.e. impulse rapping 0r vibration. For convenience the term dislodger mechanism will be used herein to apply to either type of apparatus.

RAPPING OPERATION Rappers can take various forms, as for example giant solenoids wherein the armature acts as an anvil, or pneumatic hammers in which air is rapidly applied to or exhausted from a piston type anvil, or motor-elevated hammers operating by gravity against the collector plates. In most cases the rapper-dislodger force is applied vertically against the plate suspension frame. As shown in FIG. 1, the rapper takes the form of a giant solenoid or linear motor 36 suitably mounted on roof 14; another solenoid would be located directly behind the illustrated solenoid, so that rapping force is transmitted simultaneously to the front most area of frame 24 and the rearmost area of the frame.

The armature 38 for each rapper motor is secured to frame 24 so that energization of the motor produces a rapid up and down motion of the frame 24 which is effective to shake dust particles from the collector plates 18. The space beneath the collector plates is commonly occupied by one or more hoppers which receive the separated dust particles and which are periodically opened at the bottom thereof to permit discharge of the hopper dust.

MULTIPLE COLLECTOR SECTIONS During the rapping operation the gas usually continues to pass through the spaces between wires 18 and collector plates 22 so that some of the dust particles which are shaken from the collector plates during the rapping cycle become reentrained in the gas stream. Therefore it is common practice to provide additional collector sections downstream from the principal dust collector section. The downstream collector sections are usually carbon copies of the principal collector section as above described. However they do not collect as much dust as the principal collector section because they collect pnly the reentrained dust and any other stray dust lPaItiClS which may escape the principal collector section. Commonly the collector is equipped with three collector sections arranged in series flow relation, the first one collecting and retaining something on the order of 65% of the dust, the next one collecting and retaining perhaps 25% of the dust, and the last one collecting and retaining perhaps of the dust.

Commonly the collector plates are formed with full length vertical bafiies protruding from their side surfaces at their front and rear edges, and also at points therebetween; in FIGS. 1 and 2 representative baflies are identified by numeral 38. These bafi'les form vertical pockets which channel the dust downwardly as it is rapped off of the collector plate surfaces. In spite of these baifles some of the dust in the principal collector section becomes reentrained in the main gas stream, but usually at a lower point such that it is subsequently collected at a lower point in the first set of collector plates or in the next set of collector plates. This action of the dust makes it undesirable for each of the three collector sections to be rapped at the same time or moment. Thus, when the principal collector plate section is being rapped it is desirable that the next two sections be operational for collecting any reentrained dust particles as well as any particles originally in the gas stream then flowing through the collector plates; this eliminates what is sometimes referred to as puffing, means puffs of dust particles flowing through the stack as a result of coincidental rapping in each of the three sections.

RAPPING CYCLE conventionally the rapping cycles for the various collector sections are controlled on a time basis, the principal collector section having rapped most frequently and the last collector sections being rapped least frequently. The time basis for rapping control has some disadvantage in that it does not take into account any variations in dust loading or possibility of coincidental rapping of all collector sections. As a result, one or more of the sections is usually rapped more frequently or less frequently than the desired optimum. In many instances continued surveillance of the equipment by technical experts is required.

The present invention proposes that each collector section be rapped after said section has accumulated a pre determined weight of dust thereon. The invention also proposes a scanner system of automatic surveillance for preventing coincidental rapping of the various collector sections.

As shown in FIG. 1, the precipitator is equipped with a combination sensor-trigger mechanism 40, said mechanism being disposed in a suitable fixed location registering with the collector plate assembly. As shown in FIG. 2, the mechanism includes a box or enclosure 42 adjustably or otherwise fixed atop the channel 28, said housing having a bracket 44 therein which is equipped with two laterally turned ears 46 and 48. Vertically aligned openings in these cars form guide surfaces for a plunger type sensor member 50 having an E-ring 52 thereon for engagement with a compression spring 54. Spring 54 is elfective to maintain plunger 50 in contact with an adjustable set screw 58 carried by the collector frame 24.

As the various collector plates 22 accumulate dust on their surfaces the added weight of the dust causes the frame 24 to be deflected downwardly, since it is yieldably supported by the springs 34; the plunger 50 follows this downward movement of the collector assembly until its cam surface 60 reaches the roller 62 carried by the spring arm 64. At that point arm 64 moves leftwardly to permit the spring-biased plunger 66 to close the contacts in the snap switch 68. The snap switch thus acts trigger to initiate the rapping sequence.

FIG. 4 CONTROL CIRCUIT FIG. 4 shows one form of control circuit which can utilize switch 68. The circuit there shown includes three similar switches, 68a, 68b, and 680 all similar to the FIG. 2 switch, and each having a sensor similar to the plunger type sensor 50 shown in FIG. 2. Switch 68a would be utilized with the first collector section, switch 68b would be utilized with the second collector section, and switch 680 would be utilized with the third collector section.

Cooperating with the three switches is a scanner means 70 which may take the form of a small synchronous electric motor 72 and a set of three contacts 74, 75 and 76. The shaft of the motor carries an electrical switch arm or rotor 77 which slowly traverses the three contacts 74, thereby to periodically complete a circuit between electric power line 78 and the particular contact then engaged with arm 77; one complete rotation of rotor 77 might require forty-five seconds as an example. In its illustrated position arm 77 connects with a circuit line 80 which energizes a relay coil 82 when the trigger switch 68:: is closed due to dust weight accumulations on its collector plate surfaces. At initial energization of coil 82 the controlled switch arm 84 is caused to open the contacts 86 and to close the contacts 88; at the same time the controlled switch arm -90 closes the normally open contacts 92, thereby energizing the rapper type dislodger 36a through contacts 74a. Contacts 74a are physically oriented to be opened and closed in syn chronism with contacts 74. Thus, switch arm 77 simultaneously closes contacts 74 and 74a. In similar fashion, the switch arm simultaneously closes contacts 7511 when it closes contacts 75 and contacts 7 6a,when it closes contact 76.

Relay contacts 86 are disposed in ,a circuit line 94 which normally energizes scanner motor 72. Therefore as contacts 86 open, the motor .72 is deenergized so that switch arm 77 remains in its illustrated position connecting the two circuit lines 78 and 80. Contacts 86 serve to start and stop the scanner motor action.

On some occasions one rap on the collector plates will be sufficient to substantially clean said plates of collected dust. On other occasions, or with stickier dusts, it may be necessary to repeat the rapping a number of times. To continue the rapping cycle beyond the first rapper energization the circuit is equipped with a time delay mechanism illustratively shown as a bimetal switch 102 and electric resistance "heater 104. At the start of the rapping cycle the bimetal switch is in the illustrated position completing a circuit through line 78, arm 77, line 80, relay coil 82 and line 110. As the cycle proceeds the heater 104 warps the bimetal 102 away from the fixed contact to open the circuit across line 110. If switch 68a is not opened when the bimetal 102 opens, then the rapping may continue until switch 68a is opened. Normally the time constant for mechanism 100 is chosen so that substantially all dust is dislodged from the plates before bimetal 102 opens; a few seconds is all that is usually required.

It will be appreciated that the time delay mechanism 100 can take various forms, and that any commercially available time delay mechanism can be employed in practicing the invention. In operation of the time delay mechanism, the opening of switch 102 deenergizes coil 82 and returns switches 84 and 90 to their illustrated positions; motor 72 is thus energized to advance rotor 77 toward the next contact 74, 75 or 76.

FIG. 4 shows a relay 82, a rapper 36, and a time delay mechanism 100 used to produce a controlled rapping of three collector sections 3611,3612, and 36. Only one of the sections can be rapped at any one moment because only one of the switches 74a, 75a or 76a is closed at any one moment.

FREQUENCY OF CLEANING THE PLATES As previously noted, it is sometimes desirable to rap the first collector section at more frequent time intervals than the next two collector sections. In the FIG. 4 circuit, should the scanner switch arrive at a particular contact 74, 75 or 76, and should the dust build up on the collector plates for that contact be so slight as not require cleaning, the respective switch 68a, 68b or 680 will be in the open condition, and motor 72 will remain energized so that scanner switch 76 will proceed on to the next contact 74, 75 or 76. The actual rapping of plates will therefore only take place when cleaning is necessary, as determined by the weight of dust accumulations. Also, the rapping cannot by coincidence occur simultaneously in two collector sections because the switch 77 can at any one moment only close one of the switches 74a, 75a, or 76a.

It will be appreciated that the rapper responds to specific predetermined dust weight accumulations. However, the dust actuation weight for each collector section need not be the same. For example, an accumulation of two thousand pounds of dust may be used as the control weight for the principal collector section, while a weight of only five hundred pounds may be used as the control weight for the last collector section. The last section might advantageously be kept fairly clean at all times because any reentrained dust will escape to the stack; the first or principal collector section could be permitted to accumulate larger quantities of dust because reentrained dust can be caught in the next sections.

The dust weight accumulation needed to actuate each switch 68 may be determined or varied by adjusting the travel of sensor 50, as by vertical adjustment of the switch 68 position via slots 69 in bracket 44, or by adjustment of set screw 58. Sensor 50 could of course be formed as an integral part of frame 24 or any one of the collector plates. However in that event there might be some calibration difliculties, some mechanical alignment difiiculties, and also some sealing problems. Preferably the sensor operates in a sealed enclosure 40 protected against ingress of dust particles.

FIG. 3 TRIGGER The sensor-trigger means can take various structural forms other than the form shown in FIG. 2. Thus, as shown in FIG. 3 the sensor can consist of a shutter plate 50' slidably disposed within a housing 40' and guided for vertical movement by channel grooves or guides 51 disposed on the opposite sidewalls of the housing. The space to the left of shutter 50 may be occupied by a small continuously energized glow lamp 53, and the space to the right of shutter 50 may be occupied by a cadmium sulfide photoelectric cell 55. The shutter plate 50' is intended to follow the movement of the collector plate assembly in the same manner as previously described plunger 50; 50' is therefore preferably biased downwardly, as by a spring (not shown) or by its own weight. At a suitable point the plate is provided with an aperture 57. When the plate has moved down a sufiicient distance the aperture transmits a sufficient percent of the light from bulb 53 to energize the cadmium sulfide cell 55. The current flow through the cell can be applied to a conventional amplifier located in the energizer circuit for relay 82. Thus the FIG. 3 mechanism can serve the same function as the FIG. 2 mechanism in the FIG. 4 control circuit.

In such event the actuator for the valve could be the sensor responsive to downward deflection of the collector assembly, and the fluid flow through the valve could be applied directly to the pneumatic rapper or to a fluid 5 switch arranged in a pneumatic control circuit.

The principal feature of interest relates to the concept of using dust weight accumulations on the collector plates to initiate the rapper control cycle. Preferably the triggers for the various collector sections are arranged so that only one set of collectors are rapped at any one moment.

The drawings show a collector of the single stage type, i.e. ionization of the particles and collection thereof take place in the same sect-ion or stage. The invention may be practiced in a two stage collector wherein ionization occurs upstream from the point of collection.

I claim:

1. An electrostatic precipitator comprising a housing adapted to have dusty gas pass therethrough; ionizer means arranged within the housing for charging dust particles in the gas stream; collector elements arranged within the housing to remove the charged dust particles from the gas stream; means yieldably supporting the collector elements so that dust accumulations on the collector elements cause said elements to be deflected downwardly in proportion to the weight of the dust accumulations thereon; means for dislodging the dust accumulations on the collector elements; and automatic control means for energizing the dislodging means; said control means including sensor-trigger means responsive to predetermined downward deflections of the collector elements, whereby the dislodging means is actuated only after the elements have become loaded with predetermined dust accumulations thereon.

2. The precipitator of claim 1 wherein the collector elements comprise at least two sets of collector plates arranged in series flow relation; said dislodging means comprising a separate dislodger mechanism for each set of collector plates; said control means comprising a separate sensor-trigger means for each dislodger mechanism, and a scanner means arranged to operably connect each trigger and dislodger mechanism in a predetermined time sequence so that only one dislodger mechanism is capable of operation at any one moment.

3. The precipitator of claim 2 wherein the sensor for 4 the downstream collector plates actuates its trigger after a lesser dust weight accumulation than the sensor for the upstream collector plates.

4. The precipitator of claim 2 wherein the control means comprises a time delay means operable at initia- 50 tion of each dislodging cycle to continue the dislodging operation for a predetermined time period irrespective of the condition of the sensor.

5. The precipitator of claim 1 wherein the sensor means comprises a cam movable vertically in response to downward deflections of the collector elements, and the trigger means comprises a snap action switch operably connected with the cam for snap-action movement when the cam reaches a predetermined position.

DENNIS E. TALBERT, 111., Primary Examiner US. Cl. X.R. 

